Aircraft gas turbine engines are subject to increasingly strict environmental regulations, including limits on undesirable exhaust emissions. Newer generation engines are designed to comply with existing and anticipated regulations. However, older generation engines were designed in an era when environmental regulations were less stringent or nonexistent. These older generation engines fail to comply with anticipated regulations and may have to be retired despite being serviceable in all other respects. Retiring an otherwise serviceable engine represents a significant economic loss to the engine's owner.
An appealing alternative to retiring an older generation engine is to extend its useful life with upgraded components designed to make the engine compliant with regulatory requirements. For example, engine exhaust emissions may be reduced by retrofitting the engine with redesigned combustion chambers and fuel injectors. The redesigned combustion chambers and injectors must satisfy the conflicting requirements of reducing oxides of nitrogen (NOx), reducing smoke, reducing unburned hydrocarbons (UHC) and ensuring stability of the combustion flame. In addition, the presence of the redesigned components should not materially degrade engine performance or operability or compromise the durability of the engine's turbines.
One approach to clean combustion is referred to as rich burn, quick quench, lean burn (RQL). The annular combustors used in many modern gas turbine engines often use the RQL combustion concept. A combustion chamber configured for RQL combustion has liner that encloses three serially arranged combustion zones--a rich burn zone, a quench zone and a lean burn zone. The rich burn zone is at the forwardmost end of the combustion chamber and receives fuel and air from fuel injectors that project into the combustion chamber. The quench zone is immediately aft of the rich burn zone and features a set of dilution holes that penetrate the liner to introduce dilution air into the combustion chamber. The lean burn zone is aft of the quench zone.
During operation, the fuel injectors continuously introduce a quantity of air and a stoichiometrically excessive quantity of fuel into the rich burn zone. The resulting stoichiometrically rich fuel-air mixture is ignited and burned to partially release the energy content of the fuel. The fuel rich character of the mixture inhibits NOx formation in the rich burn zone and resists blowout of the combustion flame during any abrupt reduction in engine power. However if the mixture is overly rich, the combustion chamber will produce objectionable quantities of smoke. Moreover, an excessively rich mixture suppresses the temperature of the combustion flame, which can promote the production of unburned hydrocarbons (UHC). Even if the fuel-air mixture in the rich burn zone is, on average, neither overly rich nor insufficiently rich, spatial variations in the fuel-air ratio can result in local regions where the mixture is too rich to mitigate smoke and UHC emissions and/or insufficiently rich to mitigate NOx emissions. Thus, the ability of the fuel injector to deliver an intimately and uniformly blended mixture of fuel and air to the combustion chamber plays an important role in controlling exhaust emissions.
The fuel rich combustion products generated in the rich burn zone flow into the quench zone where the combustion process continues. Jets of dilution air are introduced transversely into the combustion chamber through the quench zone dilution holes. The dilution air supports further combustion to release additional energy from the fuel and also helps to consume smoke (by converting the smoke to carbon dioxide) that may have originated in the rich burn zone. The dilution air also progressively deriches the fuel rich combustion products as they flow through the quench zone and mix with the dilution air. Initially, the fuel-air ratio of the combustion products changes from fuel rich to approximately stoichiometric, causing an attendant rise in the combustion flame temperature. Since the quantity of NOx produced in a given time interval increases exponentially with flame temperature, substantial quantities of NOx can be produced during the initial quench process. As the quenching continues, the fuel-air ratio of the combustion products changes from approximately stoichiometric to fuel lean and the flame temperature diminishes. However until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high and considerable quantities of NOx continue to form. Accordingly, it is important for the quenching process to progress rapidly to limit the amount of time available for NOx formation, which occurs primarily while the mixture is at or near its stoichiometric fuel-air ratio.
The deriched combustion products from the quench zone flow into the lean burn zone where the combustion process concludes. Additional jets of dilution air may be introduced transversely into the lean burn zone. The additional dilution air supports ongoing combustion to release energy from the fuel and helps to regulate the spatial temperature profile of the combustion products.
A low emissions combustion chamber intended as a replacement for an existing, high emissions combustion chamber in an older generation engine must also be physically and operationally compatible with the host engine. Obviously, the replacement combustion chamber must be sized to fit in the engine and should be able to utilize the engine's existing combustion chamber mounts. Furthermore, the replacement combustion chamber should not degrade the engine's performance, operability or durability. Accordingly, the quantity and pressure drop of dilution air introduced into the replacement combustion chamber should not exceed the quantity and pressure drop of dilution air introduced into the existing combustion chamber. Otherwise the operating line of the engine's compressor could rematch (shift), making the compressor susceptible to aerodynamic stall. In addition, introducing an increased quantity of dilution air into the combustion chamber would compromise the durabilty of the engine's turbines by diminishing the quantity of air available for turbine cooling. Finally, the spatial temperature profile of combustion gases entering the turbine should be unaffected by the presence of the replacement combustion chamber. Similarity of the temperature profile is important since the design of the engine's turbine cooling system, which cannot be easily modified, is predicated on the temperature profile produced by the existing combustion chamber. Any change in that profile would therefore compromise turbine durability.
The fuel injectors used in an RQL combustion chamber may be hybrid injectors. A hybrid injector includes a central, pressure atomizing primary fuel nozzle and a secondary airblast injector that circumscribes the primary nozzle. The pressure atomizing primary nozzle operates at all engine power settings including during engine startup. The airblast portion of the injector is disabled during engine startup and low power operation but is enabled for higher power operation. During operation, the primary nozzle introduces a swirling, conical spray of high pressure primary fuel into the combustion chamber and relies on an abrupt pressure gradient across a nozzle discharge orifice to atomize the primary fuel. The airblast portion of the injector introduces swirling, coannular streams of inner air, secondary fuel and outer air into the combustion chamber with the secondary fuel stream radially interposed between the air streams. Shearing action between the secondary fuel stream and the coannular air streams atomizes the fuel.
As already noted, the ability of the fuel injector to deliver an intimately and uniformly blended mixture of fuel and air to the combustion chamber is important for controlling exhaust emissions. However some spatial nonuniformity of the fuel-air ratio may be beneficial. For example, it may be desirable to have an enriched core of intermixed fuel and air near the injector centerline to guard against flame blowout during abrupt reductions in engine power. However, an overly enriched core may produce unacceptable smoke emissions during high power operation. This is especially true if the dilution air jets introduced in the combustion chamber dilution zone are unable to penetrate to the enriched core and consume the smoke.
One shortcoming of all types of turbine engine fuel injectors is their susceptibility to formation of coke, a hydrocarbon deposit that accumulates on the injector surfaces when the fuel flowing through the injector absorbs excessive heat. In a hybrid injector, coke that forms at the tip of the primary nozzle, near its discharge orifice, can corrupt the conical spray pattern of fuel issuing from the orifice so that the fuel is nonuniformly dispersed. The nonuniform fuel dispersal can result in appreciable spatial variation in the fuel air ratio, making it difficult to control NOx emissions without producing excessive smoke or UHC's in the combustion chamber rich burn zone. In extreme cases, the coke deposits may reduce the cone angle of the primary fuel spray, which can interfere with reliable ignition during engine startup.
Coke can also form on some surfaces of the airblast portion of the injector, particularly those surfaces most proximate to the combustion chamber. These deposits, like those that form at the tip of the primary nozzle, can interfere with uniform dispersal of the annular fuel and air streams. Moreover, these deposits can break away from the injector during engine operation and cause damage to other engine components.
From the foregoing it is evident that the strategy for minimizing NOx production and ensuring resistance to flame blowout (rich, low temperature burning) conflicts with the strategy for mitigating smoke and UHC's (leaner, higher temperature burning). It is also apparent that these conflicting demands are easier to reconcile if the fuel injectors provide a uniformly and intimately blended fuel-air mixture to the combustion chamber. However, an enriched core of fuel and air near the injector centerline is desirable to guard against flame blowout during abrupt engine power transients. It is also apparent that a rapid transition from a fuel rich stoichiometry to a fuel lean stoichiometry is highly desirable for inhibiting NOx formation. Finally, it is also clearly desirable that the performance or durability of the engine not be affected by the presence of replacement hardware.